There are two parts to the proposed research: treatment and basic. Prior treatment-research has shown larger variability among autistic children in response to behavior therapy, with maximal gains associated with the youngest children. Proposed treatment-research will assess the effectiveness of behavior modification with very young (before 30 months) autistic children. This research will be run as a (treatment-no treatment control) group design. The Basic research focuses on perceptual and attentional problems in autism, which relates to our finding on overselection attention (stimulus overselectivity), the extent to which autistic children experience a restriction in the number of stimulus elements in a complex input which acquire functional control over their behavior. We will investigate such stimulus overselectivity across and within sensory modalities, in social situations and as related to IQ levels in retardates and chronological age in normals. Finally, we will assess the extent to which stimulus overselectivity interferes with prompt fading procedures, stimulus generalization and observational learning.